Re: possible brw_page optimization

[Chuck Lever <cel@monkey.org>]

On Wed, 26 Jan 2000, Stephen C. Tweedie wrote:
> On Fri, 21 Jan 2000 15:21:33 -0500 (EST), Chuck Lever <cel@monkey.org>
> said:
> > i've been exploring swap compaction and encryption, and found that
> > brw_page wants to break pages into buffer-sized pieces in order to
> > schedule I/O.  
> 
> brw_page is there explicitly to perform physical block IO to disk.  If
> you want to do compression or encription, I'd have thought you want to
> do that at a higher level.

yes, i want to make the policy decisions and do the encryption at the
rw_swap_page_base() level.  the decryption/decompression would be handled
by the exit routine.

however, somehow i'd have to guarantee that all buffers associated with a
page that is to be compressed/encrypted are read/written at once.  using a
bounce page to handle the ciphertext/compressed page might be enough to do
that, since it would have no buffers already associated with it.

however, i was wondering if the optimization i did was of general use. as
i mentioned, i don't see any place that invokes brw_page() in such a way
as to trigger the logic to read only some of the buffers.

> The clean way to do this would be to provide
> a virtual file to swap over, and to allow rw_swap_page_base() to pass
> the page read or write to that file's inode's read_/write_page methods.
> Then you can do any munging you want on the virtual swap file without
> polluting the underlying swap IO code.

using a unique swap file/device makes it easy to tell when you need to
decrypt a page.  :)

	- Chuck Lever
--
corporate:	<chuckl@netscape.com>
personal:	<chucklever@netscape.net> or <cel@monkey.org>

The Linux Scalability project:
	http://www.citi.umich.edu/projects/linux-scalability/

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